Process and apparatus for spray drying or spray cooling

ABSTRACT

In a process and an apparatus (1) for spray drying or spray cooling of the so-called Filtermat® type, a more uniform product layer (7) is achieved by collecting and/or treating the product on a screen means (8, 14, 19) providing a pressure difference being at least 1.5, preferably from 1.5 to 30 and particularly from about 2 to about 20 times that provided by the product layer, when collected.

This invention relates to a process and an apparatus for spray drying orspray cooling, particularly for spray drying liquids containing solidmaterials dissolved and/or suspended therein or for spray coolingcongealable liquids.

Apparatuses for spray drying or spray cooling comprising a spray dryingor spray cooling chamber, means for atomizing a feed stream into saidspray drying or spray cooling chamber, means for bringing a drying orcooling gas into contact with said atomized feed stream in said chamberso as to provide partially dried or congealed particles, a screen meanshaving a movable surface for collecting said partially dried orcongealed particles so as to form a product layer, means for drawingdrying or cooling gas through said product layer and said screen meansin one or more drying or cooling stages, a pressure difference therebybeing applied over the screen means and the product layer, and means formoving said movable surface are known in the art.

For instance, spray drying apparatuses of this kind and processes forspray drying, which can be carried out in such apparatuses, are knownfrom U.S. Pat. Nos. 3,520,066, 3,615,723, 3,741,273 and 4,351,849, thecontents of which are incorporated herein by reference. A commerciallyused process of this kind is the so-called Filtermat® process.

Also apparatuses and processes, wherein a coating or similar treatmentis carried out concurrently with the spray drying by means of bringingthe atomized feed stream into contact with an atomized stream of acoating substance, or the like, in the spray drying chamber are known,cf. e.g. U.S. Pat. No. 4,784,878, the contents of which are incorporatedherein by reference.

In carrying out a process of the abovementioned kind, the feed stream isatomized using such means as nozzle atomizers (e.g. pressure nozzles orfluid-atomizing nozzles) or rotary atomizers (e.g. atomizer wheels oratomizer discs) and brought into contact with a stream of drying orcooling gas (laminar and/or turbulent) and possibly a coating substanceand/or other additive(s), so as to obtain partially dried or congealedparticles, which are collected as a product layer of at least partiallyagglomerated, particles on the movable surface of the screen means, suchas a perforated drum, a conveyor belt made from wire screen or a wovenfilter web, and subjected to further drying and/or cooling by drawingdrying or cooling gas through the product layer and the screen means.The further drying or cooling may take place in one or more stages, thefirst one normally being carried out in connection with the collectionof the particles on the screen means in the spray drying or spraycooling chamber, whereby drying or cooling gas is drawn through theproduct layer and the screen means. By means of the movable surface ofthe screen means, the collected product layer may be moved out of thespray drying or spray cooling chamber during the drying or cooling, orbatchwise after a desired degree of drying or cooling has been obtained.Possible further drying and/or cooling steps may be carried out infurther stages inside and/or outside the spray drying or spray coolingchamber, whereby the movable surface of the screen means can be used formoving the product layer from one stage to another.

With the purpose of achieving an improved distribution of the particlesin the product layer, particularly with regard to achievement of a layerof adequate structure and even thickness in a direction transversal tothe direction of movement of the movable surface, when using large orcylindrical drying chambers, it has been proposed to provide a turbulentzone between the chamber and the conveyor belt, cf. WO-A-89/09372. Thissolution more or less solves the problem of creating a layer of settledparticles with an even thickness in the transverse direction of theconveyor belt, does, however, create other problems as, due to arelatively high gas velocity and because of introduced supplementaryprocess air, the particles settle as a compact layer presentingsubstantive resistance to gas penetration, resulting in a decreaseddrying velocity and an increased risk of the layer of collectedparticles and the supporting movable perforated surface becomingblocked.

In order to solve this problem, in EP 0 432 207 B1, it is suggested toremove a fraction of the drying gas from the lower end of the dryingtower and possibly also to introduce a gas in the lower end of the towerat certain locations, with the purpose of controlling the stream of airdirected towards the conveyor belt, and thereby form a loose and porouslayer with a uniform distribution.

However, despite the above mentioned efforts, variations in thethickness and structure of the settled layer may still occur.

The present invention provides an improvement of the above prior art,which can be adopted independently of the above-mentioned approaches orin combination therewith as, on one hand, it can assist in obtaining auniform layer of particles and, on the other hand, is capable ofreducing the adverse effect of any variation in the thickness anddensity of the layer on the degree of drying or cooling.

Thus, the invention relates to a process for spray drying or spraycooling, wherein partially dried or congealed particles are collected ona movable surface of a screen means, thereby forming a product layer onsaid movable surface, which layer is dried or cooled in one or morestages by passing drying or cooling gas through said product layer andsaid screen means, a pressure difference being applied over the screenmeans and the product layer, and the movable surface being moved, whichprocess is characterized in the pressure difference over the screenmeans being such in at least one section of at least one drying orcooling stage of the process, that the ratio between said pressuredifference and the pressure difference over the product layer, whencollected, is at least 1.5, preferably in the range from 1.5 to 30, andparticularly in the range from about 2 to about 20.

The pressure difference over the product layer to be taken intoconsideration, when calculating the abovementioned ratio, is thepressure difference over said layer, when collected, typically ascollected at the end of the spray drying or spray cooling chamber.

As in the prior art, the process according to the invention may becombined with a coating step, or the like, if desired.

In the past, it has generally been attempted to use a screen meansproviding as low a pressure difference as possible in order to reducethe energy consumption. However, it has now been found that particularadvantages can be achieved by using a screen means, providing asubstantive pressure difference compared to the pressure difference overthe collected product layer, in at least one section of at least onedrying or cooling stage of the process.

If a screen means providing a less substantive pressure differencecompared to the product layer is used, as in the prior art, there is aconsiderable risk that the product layer may become over-treated atlocations of reduced thickness or density, whereby the treated particlesmay become damaged in these areas. E.g. over-drying may result in thedried particles becoming miscoloured and/or the taste of a food productbecoming impaired. Similarly, there is a considerable risk, that theproduct layer may become under-treated at locations of increasedthickness or density, e.g. resulting in an insufficient drying of theparticles. If, on the other hand, the pressure difference over thescreen means is substantive compared to the pressure difference over thecollected product layer, as is the case in the present invention, theeffect of any non-uniformity of the thickness or density of thecollected layer will be reduced.

By increasing the pressure difference over the screen means, twoadvantages may be obtained.

Thus, by increasing the pressure difference over the screen means in theparticle collection stage, a more uniform product layer can be obtained.Accordingly, in a preferred embodiment of the invention, the ratiobetween the pressure difference over the screen means and the pressuredifference over the product layer, when collected, is at least 1.5,preferably in the range from 1.5 to 30, and particularly in the rangefrom about 2 to about 20, in the particle collection stage.

Furthermore, by increasing the pressure difference over the screen meansin e.g. a drying step, the harmful effect of a possible non-uniformityof the collected product layer can be reduced. Accordingly, in a furtherpreferred embodiment of the invention, the ratio between the pressuredifference over the screen means and the pressure difference over theproduct layer, when collected, is at least 1.5, preferably in the rangefrom 1.5 to 30, and particularly in the range from about 2 to about 20,in a stage being an after-treatment stage carried out outside theparticle collection stage.

Usually, as the case may be, some drying or cooling of the collectedproduct layer takes place already in the particle collection stage as aresult of the drying or cooling gas passing through the collected layer,which means that the combined advantages of a more uniform product layerand a more uniform treatment can be obtained by increasing the pressuredifference over the screen means in the particle collection stage asexplained above.

Although the increased pressure difference over the screen means willresult in a more uniform structure and thickness of the product layerwithin a given area, the shape of e.g. the spray drying or spray coolingchamber may be such that it tends to provide an inherent difference inthickness of the product layer from one area to another. Thus, acircular chamber, and particularly a large circular chamber, will tendto provide a product layer being thicker in the central part of amovable surface, located in the bottom of said chamber, than in theborder areas of the movable surface. This tendency can be counteractedby adapting the screen means to provide a higher pressure difference inthe central part of the movable body than in the border areas.Accordingly, in a preferred embodiment of the invention the pressuredifference over the screen means varies across the movable surface in adirection being transversal to the direction of movement of saidsurface.

It may also be desired, that the pressure difference over the screenmeans varies across the movable surface in the direction of movement ofsaid surface. E.g. it may be desired that the screen means provides ahigher pressure difference in the area, where the collection of thepartially dried or congealed particles on the movable surface starts, inorder to reduce the amount of gas and/or particles passing through thescreen means.

The collected product layer may be moved out of the spray drying orspray cooling chamber during the drying or cooling, or batchwise after adesired degree of drying or cooling has been obtained, the former beingpreferred in case of large scale installations whereas the latter may bemore adequate in case of small scale installations, such asinstallations for laboratory or test purposes. When moved out of thechamber during the drying or cooling, the movement may be continuous orstepwise.

In preferred embodiments of the invention, the ratio between thepressure difference over the screen means, and the pressure differenceover the product layer, is in the range from about 2.5 to about 15, andparticularly in the range from about 3 to about 10.

The pressure difference over the screen means can be provided indifferent ways. E.g. it can mainly be provided by a screen member byappropriate selection of said screen member, such as by using a gaspermeable conveyor belt of suitable porosity, or the pressure differenceover the screen means can, in part, be provided by a screen member andin part by a restriction member, such as a perforated plate, located inthe immediate vicinity of the screen member on the side of the screenmember being opposite to the product layer. Alternatively, the pressuredifference can mainly be provided by a restriction member. The fact thatthe restriction member is located in the immediate vicinity of thescreen member means, that the restriction of the gas flow provided bythe restriction member is transferred to the screen member on an"essentially area-to-area" basis. On the other hand, it does not meanthat no other elements, such as supporting elements, may be presentbetween the screen member and the restriction member, as long as the"essentially area-to-area" basis is maintained, and the other elementsdo not result in an unacceptable building up of product material betweenthe screen member and the restriction member.

Variations in the pressure difference over the screen means may e.g. beobtained by using restriction members providing different restrictionsin different areas of the screen means.

If a restriction member is used, it will normally be a stationary, butexchangable member, such as a perforated plate, which may be removed forcleaning or exchanged according to product and process requirements.Furthermore, the restriction member may be adjustable, e.g. bycomprising two perforated plates, which may be mutually displaced toadjust the restriction. Various perforations and perforation patternsmay be used, such as various patterns of slits, gills, circular holes,rhombic holes etc., just as combinations of different types of holes maybe used. Also a lamellar structure or the like can serve as restrictionmember. In this case, the restriction can be adjusted by adjustment ofthe angle of the lamellae.

When a restriction member is used, the ratio between the pressuredifference over the screen member, and the pressure difference over therestriction member, is usually in the range from about 0.025 to about 2.

The screen member can e.g. be a gas permeable conveyor belt, such as aconveyor belt made from wire screen or a woven filter web, such as thedouble layer woven fabric described in U.S. Pat. No. 4,116,756, thecontents of which are incorporated herein by reference. A presentlypreferred material for such belts is a polyester material. The belt maybe tailored to provide different pressure differences in different areasby modification of the web structure. A further material, which can beuseful for the screen member/means is a sintered porous material, suchas a sintered porous metal, ceramic, and/or polymer material. Also aperforated drum can serve as screen member/means in connection with theinvention.

The invention also provides an apparatus for spray drying or spraycooling comprising a spray drying or spray cooling chamber, means foratomizing a feed stream into said spray drying or spray cooling chamber,means for bringing a drying or cooling gas into contact with saidatomized feed stream in said chamber so as to provide partially dried orcongealed particles, a screen means having a movable surface forcollecting said partially dried or congealed particles so as to form aproduct layer, means for drawing drying or cooling gas through saidproduct layer and said screen means in one or more drying or coolingstages, a pressure difference thereby being applied over the screenmeans and the product layer, and means for moving said movable surface,which apparatus is characterized in that the screen means is adapted toprovide a pressure difference being such in at least one section of atleast one of said one or more drying or cooling stages, that the ratiobetween said pressure difference and the pressure difference over theproduct layer, when collected, is at least 1.5, preferably in the rangefrom 1.5 to 30, and particularly in the range from about 2 to about 20.

In the present context, and in particular in relation to the definitionof the invention, the expression "drawn" should be understood ascomprising any transport of the gas through the product layer and thescreen means as a result of a higher pressure on the product layer sideof the screen means and the product layer than on the opposite side ofthe screen means, e.g. as obtained by suction from the opposite side ofthe screen means.

The processes and the apparatuses according to the invention areparticularly suitable for drying of sticky, hygroscopic, thermoplastic,heat sensitive, or slowly crystallizing products into free-flowingagglomerated powders and for cooling of liquids presenting similarproblems by congealing. E.g. the process and the apparatus according tothe invention may be used for drying or cooling of food products,chemicals, pharmaceuticals, detergents or auxiliary products. Forfurther non-limiting examples of products, which can be treated usingthe process and the apparatus according to the invention, particularreference is made to the aforementioned U.S. Pat. No. 4,351,849, thecontents of which are incorporated herein by reference.

In the following, the principle of the invention will be furtherexplained with reference to the accompanying drawings, with particularreference to spray drying, however, it should be understood that theprinciple will be equally applicable to spray cooling.

In the drawings

FIG. 1 is a very schematic sectional elevation of a spray dryingapparatus of the Filtermat® type, embodying the principle of theinvention, and

FIG. 2 is a similar illustration of a similar spray drying apparatus,using a different screen means construction.

Referring to FIG. 1, a spray drying apparatus indicated generally by 1is shown, which comprises a spray drying chamber 2, into which a feedstream supplied through conduits 3 is atomized through pressure nozzles4, and brought into contact with a drying gas supplied to the dryingchamber through a conduit 5 and annular openings 6 surrounding conduits3. By the contact with the drying gas, the atomized feed stream isconverted into sticky, partially dried particles, which are collected asa layer 7 on the surface of a moving belt 8, forming the bottom ofchamber 2, by suction through a plenum 10.

By the suction, a pressure difference over the belt of about 20 mm H₂ Oand a pressure difference over the collected layer, leaving the chamber,of about 10 mm H₂ O is established. Thus, the ratio between the pressuredifference over the screen means and the pressure difference over theproduct layer is about 2.0 in the spray drying chamber 2. Below themoving belt 8, at the place where it enters chamber 2, and in theimmediate vicinity thereof, a perforated plate 9 is provided, whichfurther increases the pressure difference over the assembly of themoving belt 8 and the perforated plate 9 so as to reduce the amount ofgas and/or particles passing through the moving belt.

From the chamber 2, the collected layer 7 is transferred to a dryingchamber 11, by means of the moving belt 8, in which chamber 11, layer 7is subjected to further drying by means of a hot drying gas, introducedin chamber 11 through a conduit 12 and withdrawn by suction through aplenum 13, located below chamber 11. In plenum 13 a further perforatedplate 14 is located below moving belt 8, in the immediate vicinitythereof. By means of this perforated plate 14, the pressure differenceover the assembly of moving belt 8 and perforated plate 14 is increased,whereby the risk of any areas of reduced thickness or density in theproduct layer 7 becoming heat-damaged due to over-drying issubstantially reduced. From drying chamber 11, the product layer 7 istransferred to a cooling chamber 15 by means of moving belt 8, in whichchamber 15 it is brought into contact with dehumidified, cooled air,supplied through conduit 16 and withdrawn through plenum 17, locatedbelow moving belt 7, beneath chamber 15. After cooling, the productleaves chamber 15 and moving belt 7 as shown schematically to the rightin the drawing.

FIG. 2 shows an embodiment, deviating from the embodiment shown in FIG.1 in the perforated plate 9 located below the moving belt 8 at theplace, where it enters chamber 2, having been substituted by arestriction member 19, comprising a number of perforated plates beinglocated below moving belt 8, in the immediate vicinity thereof, whichtogether cover the bottom area of spray drying chamber 2. Furthermore,openings 26 in the ceiling of the chamber 2 are provided for theintroduction of supplementary drying gas, supplied through conduit 25,inter alia with the purpose of reducing deposits on the ceiling. Inother respects, the embodiment shown in FIG. 2 corresponds to that shownin FIG. 1.

In the above, the invention has been explained by means of variousspecific embodiments thereof, however, it will be appreciated, thatvarious modifications and alterations can be made by a person skilled inthe art, without deviating from the spirit and scope of the invention.

What is claimed is:
 1. A process, wherein partially dried or congealedparticles are collected on a movable surface of a screen means, therebyforming a product layer on said movable surface, which layer is dried orcooled in one or more stages by passing drying or cooling gas throughsaid product layer and said screen means, a pressure difference beingapplied over the screen means and the product layer, and the movablesurface being moved, characterized in the pressure difference over thescreen means being such in at least one section of at least one dryingand cooling stage of the process, that the ratio between the pressuredifference over the screen means and the pressure difference over theproduct layer, when collected, is at least 1.5.
 2. A process accordingto claim 1, wherein the pressure difference over the screen means variesacross the movable surface in a direction being transversal to thedirection of movement of said surface.
 3. A process according to claim1, wherein the pressure difference over the screen means varies acrossthe movable surface in the direction of movement of said surface.
 4. Aprocess according to claim 1, wherein said ratio is in a particlecollection stage.
 5. A process according to claim 1, wherein said ratiois a stage being an after-treatment stage carried out outside a particlecollection stage.
 6. A process according to claim 1, wherein said ratiois in the range from about 2.5 to about
 15. 7. A process according toclaim 1, wherein the pressure difference over the screen means ispartially provided by a screen member, and partially by a restrictionmember being located in the immediate vicinity of the screen member onthe side of the screen member being opposite to the product layer.
 8. Aprocess according to claim 7, wherein the ratio between the pressuredifference over the screen member and the pressure difference over therestriction member is in the range from about 0.025 to about
 2. 9. Aprocess according to claim 1, wherein the pressure difference over thescreen means is mainly provided by a screen member.
 10. A processaccording to claim 7, wherein the screen member is a gas permeableconveyor belt.
 11. A process according to claim 1, wherein the pressuredifference over the screen means is mainly provided by a restrictionmember.
 12. A process according to claim 7, wherein the restrictionmember is stationary.
 13. A process according to claim 7, wherein therestriction of the restriction member is adjustable.
 14. A processaccording to claim 7, wherein the restriction member comprises aperforated plate.
 15. A process according to claim 1, wherein themovable surface is provided by a curved surface of a rotary drum.
 16. Anapparatus comprising:one of a spray drying and spray cooling chamber(2); means (4) for atomizing a feed stream into said spray drying andspray cooling chamber; means (6,26) for bringing one of a drying andcooling gas into contact with said atomized feed steam in said chamberso as to provide one of partially dried and congealed particles; screenmeans (8, 9, 14, 19) having a movable surface for collecting said onepartially dried and congealed particles so as to form a product layer(7); means for drawing one of drying and cooling gas through saidproduct layer and said screen means in one or more drying and coolingstages (2, 11, 15), a pressure difference thereby being applied over thescreen means and the product layer; and means for moving said movablesurface, wherein the screen means (8, 9, 14, 19) provides a pressuredifference whereby in at least one section of at least one of said oneor more drying and cooling stages (2, 11, 15), the ratio between thepressure difference over the screen means and the pressure differenceover the product layer (7), when collected, is at least 1.5.
 17. Anapparatus according to claim 16, wherein the screen means provides saidpressure difference over said screen means, which varies across themovable surface in a direction being transversal to the direction ofmovement of said surface.
 18. An apparatus according to claim 16,wherein the screen means provides said pressure difference over saidscreen means, which varies across the movable surface in the directionof movement of said surface.
 19. An apparatus according to claim 16,wherein the screen means provides said ratio is in the particlecollection stage.
 20. An apparatus according to claim 16, wherein thescreen means provides said ratio is in a stage being an after-treatmentstage carried out outside a particle collection stage.
 21. An apparatusaccording to claim 16, wherein said ratio is in the range from about 2.5to about
 15. 22. An apparatus according to claim 16, wherein thepressure difference over the screen means is provided by a screen member(8) and by a restriction member (9, 14, 19) being located in theimmediate vicinity of the screen member on the side of the screen memberbeing opposite to the product layer.
 23. An apparatus according to claim22, wherein the screen member and the restriction member are adapted toprovide a ratio between the pressure difference over the screen memberand the pressure difference over the restriction member in the rangefrom about 0.025 to about
 2. 24. An apparatus according to claim 16,wherein the pressure difference over the screen means is mainly providedby a screen member.
 25. An apparatus according to claim 22, wherein thescreen member is a gas permeable conveyor belt.
 26. An apparatusaccording to claim 16, wherein the pressure difference over the screenmeans is mainly provided by a restriction member.
 27. An apparatusaccording to claim 22, wherein the restriction member is stationary. 28.An apparatus according to claim 22, wherein the restriction of therestriction member is adjustable.
 29. An apparatus according to claim22, wherein the restriction member comprises a perforated plate.
 30. Anapparatus according to claim 16, wherein the movable surface is providedby a curved surface of a rotary drum.
 31. The process according to claim1, wherein said ratio is in the range of about 1.5 to about
 30. 32. Theprocess according to claim 1, wherein said ratio is in the range fromabout 2 to about
 20. 33. A process according to claim 9, wherein thescreen member is a gas permeable conveyor belt.
 34. A process accordingto claim 11, wherein the restriction member is stationary.
 35. A processaccording to claim 11, wherein the restriction of the restriction memberis adjustable.
 36. A process according to claim 11, wherein therestriction member comprises a perforated plate.
 37. An apparatusaccording to claim 24, wherein the screen member is a gas permeableconveyor belt.
 38. An apparatus according to claim 26, wherein therestriction member is stationary.
 39. An apparatus according to claim26, wherein the restriction of the restriction member is adjustable. 40.An apparatus according to claim 26, wherein the restriction membercomprises a perforated plate.
 41. A process according to claim 1,wherein said ratio is in the range from about 3 to about
 10. 42. Anapparatus according to claim 16, wherein said ratio is in the range from1.5 to
 30. 43. An apparatus according to claim 16, wherein said ratio isin the range from about 2 to about
 20. 44. An apparatus according toclaim 16, wherein said ratio is in the range from about 3 to about 10.